1. Field of the Invention
This invention relates to coolable wall elements and more specifically to elements combining impingement and transpiration cooling techniques.
2. Description of the Prior Art
A limiting factor in high temperature machinery, such as gas turbine engines, is the maximum temperature of the working medium gases which can be tolerated in the machine without adversely limiting the durability of the individual components. Specifically, within gas turbine engines the rotor blades and the nozzle guide vanes of the turbine are susceptible to thermal damage and are cooled by a variety of techniques. Nearly all known techniques utilize air which is bled from the compressor and flowed to the local area to be cooled through suitable conduit means.
The range of techniques which have been proposed in the past and those which are being proposed today continue to emphasize reduced cooling air consumption and improved cooling effectiveness. Impingement cooling is known to be one of the more effective techniques for efficiently utilizing cooling air. In impingement cooling a high velocity stream of air is directed against the component to be cooled. The high velocity stream impinges upon a surface of the component and increases the rate of heat transfer between the component and the cooling air. A typical application of impingement cooling is discussed by Smuland et al in U.S. Pat. No. 3,628,880 entitled "Vane Assembly and Temperature Control Arrangement". Smuland et al shows baffle plates interposed between the cooling air supply and the component to be cooled. Orifices in each plate direct jets of the cooling air across an intermediate space between the baffle and the cooled component during operation of the engine. The pressure ratio across each plate is sufficiently high to cause the cooling air flowing through the plate to accelerate to velocities at which the flow impinges upon the opposing surface of the component to be cooled. Cooling air is exhausted from the intermediate space between the plate and the opposing surface at a high rate to prevent the buildup of back pressure within the space. In Smuland et al film cooling passageways are utilized to exhaust the impingement flow.
A second highly effective, but not as widely utilized technique, is that of transpiration cooling. A cooling medium is allowed to exude at low velocities through a multiplicity of tiny holes in the wall of the component to be cooled. The low velocity flow adheres to the external surface of the component to isolate the component from the heat source. In transpiration cooling the exuding velocities remain low in order to prevent overpenetration of the working medium gases by the cooling air. Overpenetration prevents adherence of the cooling fluid to the component and interrupts the flow of medium gases. One typical application of transpiration cooling to a turbine vane is discussed by Moskowitz et al in U.S. Pat. No. 3,706,506 entitled "Transpiration Cooled Turbine Blade with Metered Constant Flow". Moskowitz et al shows a plurality of coolant channels formed across the chord of the blade to accommodate both temperature and pressure gradients across the chord. Cooling air is flowed to each channel through a metering plate at the base of the airfoil section. A preferred pressure ratio across the cooled wall in most transpiration cooled embodiments is approximately 1.25. The effectiveness of the transpiration cooled construction is highly sensitive to variations from the design pressure ratio across the surface to be cooled; accordingly, the pressure ratio must be closely controlled. Impingement and transpiration cooling are incorporated in one airfoil section in U.S. Pat. No. 3,726,604 to Helms et al entitled "Cooled Jet Flap". The impingement cooling is applied to the leading edge of the airfoil section and transpiration cooling is applied to the suction and pressure walls; however, both cooling techniques are not applied simultaneously to supplement each other in cooling a common portion of the vane wall.
The above-described individual cooling techniques, have been successful in prolonging the life of various machine components. A requirement for even more durable, high performance machinery, however, exists. More effective techniques for cooling with lesser quantities of air are continually being sought.